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ok if im not mistaken it will not work because in order to ignite fuel in a spud gun you need a electris bridge of electricity... if you switch the main contact coming from the sparker or other ignition source...the spark will jump to ANY mettal nearby. which means that all four will go at ounce instead of the selected cannon....i think thats how it is but go ahead an try to see if it works...first thing i would say if you will try this is to build a mini version of the selector switch with the 4 optionl cannons and switch them one by one and if you see the electricity bridge jump to the nearby contact it will not work cuz if you do it like that it will just be igniting all 4 at ounce no matter what u do...

correct if i am wrong but in the thread above he Did ignite fuel without the said "Bridge of electricity". And the spark will not jump to Any metal nearby, because just like in any electric circuit you need a complete circle of electricity.

Brplatz: Yes, the resistive ignitor will work. You need a heck of a lot of current but if you have several RC batteries that should be enough.

Back to the original high voltage siwitch idea. Just get a multiposition switch from radioshack and give it a try. If you keep all the spark gaps in the guns small then there is a good chance you'll be able to get it to work like you want. Since a combustion gun really doesn't care how big the spark gap is you can just use very small gaps (less than 1/16") which should be less than the gaps in the selector switch.

Building a switch wouldn't be all that hard. Start with a piece of copper clad board (from Radioshack). Draw a pattern on the board with a Sharpie marker, etch in 3% hydrogen peroxide + Chlorox bleach. Make the rotor from a choped up coffee can on a bolt axle. If you want to get fancy, add a couple holes to the board and whole in the rotor that are a bit smaller than a BB. Use the BB as the detent mechanism.

i have yet to find a 50,000 v selector switch. the closest to it is a c-switch. you know, the switches that you see in frankenstein's monster. anyway they have huge gap distances which make them ideal for hv. but remember you should, insulated or not, always have a switch gap that is larger than an arc can jump because insulation degrades over time no matter how much you apply. mineral oil slows the process down but there is not technology to prevent it entirely. also watch out for corona leaks. if the make o3, which is poisonous, it will cause premature arcing. the simplest way to cut down on corona is to reduce the numbers of corners on the conductor and to "quench" the gap with an air flow, constantly removing corona and o3.

But you don't need a 50KV selector switch. You need a selector switch rated at higher voltage than your spark gap(s). An 1/8" gap would only need a switch rated to 125~400V (that's the voltage rating when the switch is open).